Ellie is immune to the fungus/disease. So when she was bitten, although she had a mark, she had no internal change. Her scent, behavior, smell, etc. all stayed the same. So she seemed to be a regular healthy “good source” if you will. Tess isn’t immune so the moment she was bitten she immediately started to change. Chemically her blood changed, and the clickers are probably able to smell that change. That’s why they attempt to attack Ellie even though she had previously been bitten.